Variable or selectable time delay is useful for applications such as phased array radar, wherein the phase of the transmitted or received signal at each antenna element is adjusted in order to steer the transmitted or received beam. Phased array radar is typically used to determine speed and direction over a period of time. An example of a prior art antenna phased array system 100 is illustrated in FIG. 1. An X-band microwave oscillator 101 operating at 10 GHz, for example, drives an array of phase shift elements 102, which shift the phase of a signal output by oscillator 101. An array of electrical amplifiers 103 increases the signal powers, and an array of antenna elements 104 radiate the amplified, phase shifted versions of the signal output by oscillator 101 into free space, creating an electromagnetic radiation phase front 105, propagating in direction 106, which is perpendicular to phase front 105.
Antenna array system 100 is called a phased array given that a multiplicity of desired directions 106 of transmission may be obtained from the antenna array through a selection of phase via phase shift elements 102 for each antenna element 104. Variable phase shifters have been used to achieve beam steering over a narrow modulation frequency band. The phase shift elements 102 may have a phase shift that varies with modulation frequency, however, resulting in a beam direction 106 that changes with modulation frequency, referred to as squint. The best way to achieve the correct phase over a broad modulation bandwidth—for example +/−10% of the 10 GHz center frequency—is to use variable or selectable time delay elements, which helps to avoid squint.
An important goal of phased array radar systems has been to obtain true time delay without degrading the source signal through introduction of noise and loss in the delay lines. In the prior art, it has been difficult, however, to achieve variable time delay electrically. Long electrical delay lines are typically used for the long time delays and high modulation frequency required by many phased array radars. Long electrical delay lines typically have both large loss and large frequency dependence.
Optical subsystems using optical beam forming have been employed in the prior art to introduce delay. For example, optical subsystems have been used to convert the electrical signal output from oscillator 101 into an optical signal, manipulate the optical signal, then return the signal to the electrical domain prior to generating the phase front 105. For example, the electrical signal output from oscillator 101 is converted into an optical signal by amplitude modulation of an optical carrier. Optical frequency or phase modulation have also been employed in the prior art.
One prior art optical beam forming system is for radar transmission systems. The U.S. Naval Research Laboratory has demonstrated a transmit beamformer that provides squint-free 60-degree steering across the entire Ka band (i.e., 26.5 GHz to 40 GHz). D. A. Tulchinsky and P. J. Matthews, Ultrawide band fiber optic control of a Millimeter wave transmit beamformer, IEEE Trans. Microwave Theory and Techniques Vol. 49, No. 7, pp 1248–1253 (July 2001). The beamformer is based on dispersive prism optical delay lines.
Another prior art optical beam forming system is for phased array radar receivers. As described by P. J. Matthews, M. Y. Frankel, and R. D. Esman in A Wideband Fiber Optic True Time-Steered Array Receiver, IEEE MIT's Digest pp. 347–350 (Jun. 8, 1997), the U.S. Naval Research Laboratory has demonstrated receiver beam forming using photonics. The receiver demonstrated 60 degree steering across the 6–18 GHz region without squint and with 8 elements. This approach uses tunable lasers with dispersive delay lines.
Prior art beam forming with multiple beams has also been demonstrated in the prior art for a phased array radar receiver. P. J. Matthews, M. Y. Frankel, and R. D. Esman, A Wide Band Fiber Optic True Time Steered Array Receiver Capable of Multiple Independent Simultaneous Beams, IEEE Photonics Technology Letters, Vol. 10, No. 5 (May 1998). For this prior art system, eight modulated optical signals are fed to an eight-channel fiber-optic dispersive prism. The prism provides a wavelength-dependent time delay for each element channel, which is proportional to the position of the corresponding element in the array. Simultaneous multiple beams on reception provide fast switching.
Another prior art beam forming system involves two-dimensional beam steering. As discussed by D. A. Tulchinsky and P. J. Matthews in Demonstration of a Reconfigurable Beamformer for Simplified 2-D, Time-Steered Arrays, 2000 IEEE MTT-S Digest, pp. 839–42 (Jun. 11, 2000), two-dimensional beam steering has been demonstrated using an 8×8 optical switch matrix to get different delays. The prior art 8×8 optical switch matrix is a commercially-available device comprised of Mach-Zehnder interferometers and electronically controlled thermo-optic phase shifters, allowing control of which of the eight input ports is solely routed to one of the eight output ports.
Another prior art approach to optical beam forming is the White cell approach described by B. L Anderson and others in Optically Produced True Time Delays for Phased Antenna Arrays, Applied Optics, Vol. 36, No. 32, pp. 8493–8503 (Nov. 10, 1997). A White cell is a three-mirror optical cell designed originally by J. White. Optical beam forming is accomplished by using a device based on the White cell. The device uses a simple optical cavity comprising three spherical mirrors that recirculates a beam many times through the cell and refocuses the beam on each path. One of the major strengths of this approach is the fact that the number of delays scale quadratically (or higher) with the number of mirror bounces. This is an advance compared with previous fiber optic delay line work wherein a separate fiber is required for each delay.